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E. B. Bierhaus (Lockheed Martin), C. R. Chapman, W. J. Merline (SwRI)
There are (at least) three significant implications from our discovery that the bulk of Europa's small crater population is comprised of secondary craters. (1) Using the more numerous, and thus statistically more significant, small craters to derive surface ages is now suspect. Far-flung secondaries from a single primary crater can dominate a local crater population, so a high-density population doesn't necessarily reflect a greater integrated exposure time, it could instead result from a single exposure to extensive crater ejecta. (2) The Jovian system's primary impacting population, which presumably is comprised of mostly comets as understood from recent dynamical arguments, is deficient in small objects. Thus, small comets are rare in the Jovian system. Because many of the Jupiter-family comets come from the Kuiper belt, it's possible that the Kuiper belt has a shallow size-distribution at small sizes. (3) Ejecta production that makes secondary craters is staggeringly efficient. There are very few large primary craters on Europa (less than 40 are currently known), yet we've identified tens of thousands of secondaries in high-resolution Galileo images that cover less than 1% of Europa's surface. Although there are mechanical differences between ice and rock during the crater excavation process, very cold ice such as that on Europa behaves similarly to rock. We expect that impacts into rocky terrain also produce similar multitudes of secondary craters. Thus we believe the small-crater record of the inner Solar System is significantly contaminated by secondary craters. Any attempt to derive impacting populations (i.e.\ near-Earth objects at diameters below what's found in telescopic surveys) based on lunar or martian small-crater curves will suffer a significant bias from the secondary population.
The author(s) of this abstract have provided an email address for comments about the abstract: edward.b.bierhaus@lmco.com
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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.